Christian Britschgi

Identification of the p53 family-responsive element in the promoter region of the tumor suppressor gene hypermethylated in cancer 1

The tumor suppressor gene hypermethylated in cancer 1 (HIC1), located on human chromosome 17p13.3, is frequently silenced in cancer by epigenetic mechanisms. Hypermethylated in cancer 1 belongs to the bric à brac/poxviruses and zinc-finger family of transcription factors and acts by repressing target gene expression. It has been shown that enforced p53 expression leads to increased HIC1 mRNA, and recent data suggest that p53 and Hic1 cooperate in tumorigenesis. In order to elucidate the regulation of HIC1 expression, we have analysed the HIC1 promoter region for p53-dependent induction of gene expression. Using progressively truncated luciferase reporter gene constructs, we have identified a p53-responsive element (PRE) 500 bp upstream of the TATA-box containing promoter P0 of HIC1, which is sequence specifically bound by p53 in vitro as assessed by electrophoretic mobility shift assays. We demonstrate that this HIC1 p53-responsive element (HIC1.PRE) is necessary and sufficient to mediate induction of transcription by p53. This result is supported by the observation that abolishing endogenous wild-type p53 function prevents HIC1 mRNA induction in response to UV-induced DNA damage. Other members of the p53 family, notably TAp73β and ΔNp63α, can also act through this HIC1.PRE to induce transcription of HIC1, and finally, hypermethylation of the HIC1 promoter attenuates inducibility by p53.

The death-associated protein kinase 2 is up-regulated during normal myeloid differentiation and enhances neutrophil maturation in myeloid leukemic cells

The death‐associated protein kinase 2 (DAPK2) belongs to a family of Ca2+/calmodulin‐regulated serine/threonine kinases involved in apoptosis. During investigation of candidate genes operative in granulopoiesis, we identified DAPK2 as highly expressed. Subsequent investigations demonstrated particularly high DAPK2 expression in normal granulocytes compared with monocytes/macrophages and CD34+ progenitor cells. Moreover, significantly increased DAPK2 mRNA levels were seen when cord blood CD34+ cells were induced to differentiate toward neutrophils in tissue culture. In addition, all‐trans retinoic acid (ATRA)‐induced neutrophil differentiation of two leukemic cell lines, NB4 and U937, revealed significantly higher DAPK2 mRNA expression paralleled by protein induction. In contrast, during differentiation of CD34+ and U937 cells toward monocytes/macrophages, DAPK2 mRNA levels remained low. In primary leukemia, low expression of DAPK2 was seen in acute myeloid leukemia samples, whereas chronic myeloid leukemia samples in chronic phase showed intermediate expression levels. Lentiviral vector‐mediated expression of DAPK2 in NB4 cells enhanced, whereas small interfering RNA‐mediated DAPK2 knockdown reduced ATRA‐induced granulocytic differentiation, as evidenced by morphology and neutrophil stage‐specific maturation genes, such as CD11b, G‐CSF receptor, C/EBPε, and lactoferrin. In summary, our findings implicate a role for DAPK2 in granulocyte maturation.

HIC1 tumour suppressor gene is suppressed in acute myeloid leukaemia and induced during granulocytic differentiation

A hallmark of acute myeloid leukaemia (AML) is a block in differentiation caused by deregulated gene expression. The tumour suppressor Hypermethylated In Cancer 1 (HIC1) is a transcriptional repressor, which is epigenetically silenced in solid cancers. HIC1 mRNA expression was found to be low in 128 patient samples of AML and CD34+ progenitor cells when compared with terminally differentiated granulocytes. HIC1 mRNA was induced in a patient with t(15;17)‐positive acute promyelocytic leukaemia receiving all‐trans retinoic acid (ATRA) therapy. We therefore investigated whether HIC1 plays a role in granulocytic differentiation and whether loss of function of this gene might contribute to the differentiation block in AML. We evaluated HIC1 mRNA levels in HL‐60 and U‐937 cells upon ATRA‐induced differentiation and in CD34+ progenitor cells after granulocyte colony‐stimulating factor‐induced differentiation. In both models of granulocytic differentiation, we observed significant HIC1 induction. When HIC1 mRNA was suppressed in HL‐60 cells using stably expressed short hairpin RNA targeting HIC1, granulocytic differentiation was altered as assessed by CD11b expression. Bisulphite sequencing of GC‐rich regions (CpG islands) in the HIC1 promoter provided evidence that the observed suppression in HL‐60 cells was not because of promoter hypermethylation. Our findings indicate a role for the tumour suppressor gene HIC1 in granulocytic differentiation. Low expression of HIC1 may very well contribute to pathogenic events in leukaemogenesis.

The tumor suppressor gene hypermethylated in cancer 1 is transcriptionally regulated by E2F1

The Hypermethylated in Cancer 1 (HIC1) gene encodes a zinc finger transcriptional repressor that cooperates with p53 to suppress cancer development. We and others recently showed that HIC1 is a transcriptional target of p53. To identify additional transcriptional regulators of HIC1, we screened a set of transcription factors for regulation of a human HIC1 promoter reporter. We found that E2F1 strongly activates the full-length HIC1 promoter reporter. Promoter deletions and mutations identified two E2F responsive elements in the HIC1 core promoter region. Moreover, in vivo binding of E2F1 to the HIC1 promoter was shown by chromatin immunoprecipitation assays in human TIG3 fibroblasts expressing tamoxifen-activated E2F1. In agreement, activation of E2F1 in TIG3-E2F1 cells markedly increased HIC1 expression. Interestingly, expression of E2F1 in the p53−/− hepatocellular carcinoma cell line Hep3B led to an increase of endogenous HIC1 mRNA, although bisulfite genomic sequencing of the HIC1 promoter revealed that the region bearing the two E2F1 binding sites is hypermethylated. In addition, endogenous E2F1 induced by etoposide treatment bound to the HIC1 promoter. Moreover, inhibition of E2F1 strongly reduced the expression of etoposide-induced HIC1. In conclusion, we identified HIC1 as novel E2F1 transcriptional target in DNA damage responses. (Mol Cancer Res 2009;7(6):916–22)

SIRT1 is downregulated during neutrophil differentiation of acute promyelocytic leukaemia cells.

Antonio Macciò Clelia Madeddu Paola Chessa Giovanni Mantovani Renzo Galanello Department of Obstetrics and Gynaecology, ‘‘Sirai’’ Hospital, Carbonia, Department of Medical Oncology, University of Cagliari, Cagliari, Pharmaceutical Unit, ‘‘San Michele’’ Hospital, Cagliari, and Paediatric Clinic 2nd, University of Cagliari-Ospedale Regionale Microcitemie, ASL 8, Cagliari, Italy. E-mail: a.maccio@tin.it

Inactivation of the hypermethylated in cancer 1 tumour suppressor--not just a question of promoter hypermethylation?

The chromosomal region 17p13.3 is frequently deleted or epigenetically silenced in a variety of human cancers. It includes the hypermethylated in cancer 1 (HIC1) gene placed telomerically to the p53 tumour suppressor gene. HIC1 encodes a transcriptional repressor, and its targets identified to date are genes involved in proliferation, tumour growth and angiogenesis. In addition, HIC1 functionally cooperates with p53 to suppress cancer development. Frequent allelic loss at position 17p13.1 in human cancers often points to mutations of the tumour suppressor p53. However, in a variety of cancer types, allelic loss of the short arm of chromosome 17 may hit regions distal to p53 and, interestingly, without leading to p53 mutations. Furthermore, the neighbouring region 17p13.3 often shows loss of heterozygosity or DNA hypermethylation in various types of solid tumours and leukaemias. In line with this concept, Wales et al. described a new potential tumour suppressor in this region and named it hypermethylated in cancer 1 (HIC1). Further, it was shown that in the majority of cases hypermethylation of this chromosomal region leads to epigenetic inactivation of HIC1. A role for HIC1 in tumour development is further supported by a mouse model, since various spontaneous, age- and gender-specific malignant tumours occur in heterozygous Hic1+/- knockout mice. Furthermore, exogenously delivered HIC1 leads to a significant decrease in clonogenic survival in cancer cell lines. This review highlights the role of HIC1 inactivation in solid tumours and particularly in leukaemia development.

Human DMTF1β antagonizes DMTF1α regulation of the p14(ARF) tumor suppressor and promotes cellular proliferation.

The human DMTF1 (DMP1) transcription factor, a DNA binding protein that interacts with cyclin D, is a positive regulator of the p14ARF (ARF) tumor suppressor. Our earlier studies have shown that three differentially spliced human DMP1 mRNAs, α, β and γ, arise from the human gene. We now show that DMP1α, β and γ isoforms differentially regulate ARF expression and promote distinct cellular functions. In contrast to DMP1α, DMP1β and γ did not activate the ARF promoter, whereas only β resulted in a dose-dependent inhibition of DMP1α-induced transactivation of the ARF promoter. Ectopic expression of DMP1β reduced endogenous ARF mRNA levels in human fibroblasts. The DMP1β- and γ-isoforms share domains necessary for the inhibitory function of the β-isoform. That DMP1β may interact with DMP1α to antagonize its function was shown in DNA binding assays and in cells by the close proximity of DMP1α/β in the nucleus. Cells stably expressing DMP1β, as well as shRNA targeting all DMP1 isoforms, disrupted cellular growth arrest induced by serum deprivation or in PMA-derived macrophages in the presence or absence of cellular p53. DMP1 mRNA levels in acute myeloid leukemia samples, as compared to granulocytes, were reduced. Treatment of acute promyelocytic leukemia patient samples with all-trans retinoic acid promoted differentiation to granulocytes and restored DMP1 transcripts to normal granulocyte levels. Our findings imply that DMP1α- and β-ratios are tightly regulated in hematopoietic cells and DMP1β antagonizes DMP1α transcriptional regulation of ARF resulting in the alteration of cellular control with a gain in proliferation.

Tumor suppressor genes in myeloid differentiation and leukemogenesis.

Tumor suppressor genes, such as p53, RB, the INK4-ARF family and PML, suppress malignant transformation by regulating cell cycle progression, ensuring the fidelity of DNA replication and chromosomal segregation, or by inducing apoptosis in response to potentially deleterious events. In myeloid leukemia, hematopoietic differentiation resulting from highly coordinated, stage-wise expression of myeloid transcription and soluble signaling factors is disrupted leading to a block in terminal differentiation and uncontrolled proliferation. This virtually always involves functional inactivation or genetic disruption of one or several tumor suppressor genes in order to circumvent their checkpoint control and apoptosis-inducing functions. Hence, reactivation of tumor suppressor gene function has therapeutic potential and can possibly enhance conventional cytotoxic chemotherapy. In this review, we focus on the role of different tumor suppressor genes in myeloid differentiation and leukemogenesis, and discuss implications for therapy.

Abstract 300: Inhibition of oncogenic MAPK signaling in melanoma triggers SOX2-dependent adaptive drug resistance

Treatment of metastatic melanoma with BRAF V600E -specific small molecule inhibitors (BRAFi) provides dramatic clinical relief response but fails to suppress long-term tumor growth in the long-term. Extensive studies have uncovered identified a number of resistance-causing genetic alterations that, which are acquired upon continued drug exposure. However, little is known about the changes in gene expression programs changes during the critical time before drug resistance is manifested on at the genomic level. These changes may lead to adaptive drug resistance and hence, represent vital mechanisms by which tumor cells survive the acute drug insult prior to acquisition of resistance-causing mutations. Using a combination of RNAseq and molecular analyses, we find that acute inhibition of oncogenic MAPK signaling by BRAFi or MEKi in established and primary patient-derived melanoma cell cultures, leads to a rapid induction of the stem cell maintenance and pluripotency factor SOX2. Immunohistochemical analysis of Sox2 expression in melanoma sections derived from Braf V600E /Pten −/− -mice treated for 36 hours with BRAFi/MEKi confirmed the emergence of a Sox2-positive melanoma subpopulation in vivo. SOX2 Ectopic ectopic expression of SOX2 protected melanoma cells from the anti-proliferative effects of BRAFi indicative ofindicating a role for SOX2 in mediating drug resistance. Furthermore, siRNA-mediated depletion or overexpression experiments of SOX2 unveiled revealed that it is necessary and sufficient for the expansion of a drug-tolerant SOX2 + CD24 + cell population. This subpopulation is characterized by increased migratory and invasive behavior as well as an increased proliferative potential in the presence of BRAFi when compared to SOX2 + CD24 − cells. These results imply that BRAFi treatment induces a SOX2-dependent cellular program with features of adaptive drug resistance. Suppression of drug-dependent induction of SOX2 or its downstream effector pathways could potentially extend the clinical efficacy of BRAFi. Citation Format: Stefanie Flueckiger, Andrea Aloia, Lukas Frischknecht, Amanda Tuozzo, Davide Croci, Olga Shakhova, Christian Britschgi, Reinhard Dummer, Mitchell Paul Levesque, Wilhelm Krek. Inhibition of oncogenic MAPK signaling in melanoma triggers SOX2-dependent adaptive drug resistance. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 300.